The invention relates to a method of preparation of a cellulosic pulp capable of being defibrated (fluffed) for the manufacture of an absorbent material intended to be incorporated as a component in absorbent products. The invention also relates to the cellulosic pulp, the absorbent material, and the absorbent product.
Chemical pulp, particularly bleached chemical pulp, and bleached and unbleached chemothermomecanically manufactured pulp, so called CTMP, are used as materials for the manufacture of absorbent materials intended to be incorporated as a main component in absorbent products, such as diapers, sanitary towels, incontinence protections and other hygiene products as well as some type of serviettes and the like.
Bleached chemical pulp has a good wettability and initially a good liquid distribution capacity and hence a high absorbency rate. On the other hand, it is difficult to defibrate properly because of its high fibre to fibre binding capacity. This is a drawback, i.e. because a better separation of fibres gives a higher network strength and hence increased strength to the absorbent product. In connection herewith should be mentioned that an increased strength is an increasingly important feature and in many cases a critical feature for applications such as napkins and sanitary towels, as there is a tendency to reduce the content of fluff pulp in such absorbent products in order to make them thinner. Often, debonders therefore are added to chemical pulp in order to improve the deliberation efficiency. This, however, impairs the liquid distribution capacity of the defibrated pulp and hence its absorbency rate, which is a drawback.
As far as CTMP is concerned, the conditions are different. That kind of pulp is not as wettable as chemical defibrated pulp is in its defibrated condition and has therefore usually not as good distribution features as the latter one. On the other hand, the defibration features are more favorable, which reduces the need of debonders.
In SE 500 871 experiments are described which show that the distribution features and the absorbency rate of CTMP fluff pulp can be improved by impregnation of the fibres in aqueous suspension with an aluminium salt in aqueous solution at a pH which preferably should lie in the range 8.5 to 9.5, particularly at pH 9. According to SE 500 858 of the same applicant, the improved distribution capacity and absorbency rate are attributed to the formation of a porous layer of hydrophilic chemicals attached to the fibre surface, which considerably increases the specific surface of the fibres is EP 500 858 it is recommended that aluminium ions are added in the form of polyaluminium chloride or sulphate, aluminium phosphate or sodium abominate or mixtures thereof, and it is also stated that polyaluminium ions appear to function more efficiently than ordinary aluminium ions. EP 500 571recommends polyaluminium chloride, polyaluminium sulphate, sodium aluminate, and other basic aluminium compounds, but describes substantially only experiments made with additions of polyaluminium chloride and polyaluminium sulphate to CTMP at pH 9. In one case, CTMP fibres were impregnated with water-glass, which was precipitated at pH 9 with alum and polyaluminium chloride. Any significant increase of the absorbency rate could not be noticed in the case of alum and nor any increase of the network strength. Only in SE 500 871 experiments with chemical pulp are described, examples 1 and 17. At these experiments, the pulp was impregnated with polyaluminium sulphate at pH 3 and pH 11, and with polyaluminium chloride at pH 9, respectively. In both cases the absorbency rate was increased. In the first case, however, not clear at which pH, also the network strength was increased to some degree: from 3.7 to 4.3 N.
It is a primary object of the invention to improve the defibration efficiency of chemical pulp in order to reduce the knot content and to increase the network strength of chemical pump by improved separation of the fibres. Due to the improved defibration efficiency also the defibration energy supplied at the defibration can be reduced and the need of debonders be eliminated or significantly reduced. It is also an object to provide a cellulosic pulp with good absorption features, including distribution features and a high absorbency rate, particularly an improved distribution capacity as compared with chemical fluff pulp, the defibration efficiency of which has been improved through the addition of organic debonder.
Experiments have also been made with CTMP, which indicate that the method of the invention can improve the absorbency rate of that type of pulp significantly, however not noteworthy its defibration efficiency in terms of reduced defibration energy and/or increased network strength.
The above objects and effects can be achieved therein that the invention is characterised by what is stated in the appending claims. Further characteristic features and aspects of the invention will be apparent from the following description of performed experiments and by the discussion in connection thereto.